The present invention relates in general to the coating arts and, more particularly, to fine powders especially suited to the generation of coatings by plasma spray techniques.
Plasma spray coating techniques are well recognized in the art and are, in fact, widely used in industry. In a typical plasma spray operation an inert gas, such as argon, is electrically excited in a suitable spray gun resulting in a high temperature plasma. The plasma temperatures may be on the order of 20,000.degree. F. and very high plasma velocities exiting the gun are possible.
Plasma spray coating procedures utilize the simple mechanism of injecting suitable coating powders in this hot, high velocity plasma stream wherein the particles are heated and propelled to the surface to be coated or where the deposit is to be formed. Because the particles are impacted at high temperature against the surface, dense adherent coatings may be achieved.
Plasma sprayed zirconia has found utility as a thermally insulative coating on certain gas turbine engine components. The typical zirconia spray powders in current use are stabilized with either calcia or magnesia, usually at about the 5 percent by weight stabilizer level. Basically, the stabilizer is used to generate and maintain the zirconia in a cubic metallographic structure for mechanical property reasons, including thermal shock resistance.
Unfortunately, although the calcia/magnesia stabilized zironcia may be readily sprayed and exhibits stability at lower temperatures, the stability of the composition at more elevated temperatures, as may be encountered in some gas turbine engine applications, is marginal.
It is also known that yttria will stabilize zirconia and will afford stability to higher temperature levels than either calcia or magnesia. However, spray trials with yttria stabilized zirconia soon reveal very low spray efficiencies with this composition, particularly in an interparticle bonding sense.